Viscosity measurement

ABSTRACT

The present invention relates to a method and apparatus of determining the rheological properties of a polymer flowing in a conduit. The invention provides a method of characterising a polymer under test, comprising: Detecting acoustic emissions from said polymer flowing in a conduit to provide acoustic emission data, comparing the acoustic emissions data obtained against acoustic emission data from a polymer, or a series of polymers, of known characteristics, and thereby characteristing the polymer.

[0001] The present invention relates to a method of determining therheological properties of a polymer flowing in a conduit. The method isparticularly suitable for determining the viscosity of the polymer, andother properties such as the molecular structure or chemical compositionof the polymer can also be determined. Also provided is an apparatus forcarrying out the method.

[0002] Polymers are generally manufactured using chemical synthesisreactions between one or more basic molecules, known as monomers, whichreact together under favourable conditions to form a polymer, whichconsists of long chains of the monomers joined together.

[0003] In general, in polymer manufacturing processes, the compositionof the polymer chain (i.e. the molecular structure of the polymer) iscarefully controlled by adding the monomer(s) to the reaction mixture ata carefully controlled rate. Where there are two or more monomers, theseare added to the reaction mixture in strictly controlled proportions toone another e.g. in a constant ratio. It is also necessary to maintainthe reaction conditions at the correct levels in order to control therate at which each monomer reacts with the other monomer(s) and hencecontrol the resulting molecular structure. Reaction conditions includetemperature, pressure, rate of mixing, rate of shear etc. Of course,even when a single monomer is used, such as in the manufacture ofpolyvinyl chloride (PVC) or polyethylene, the molecular structure isalso affected by the reaction conditions because the length of eachpolymer chain can vary and the chains can be branched or unbranched tovarying degrees. This degree of “branching” of the polymer chain affectsthe physical properties (e.g. density and strength) of the polymerproduct.

[0004] Clearly then, the molecular structure of a polymer product mustbe carefully controlled during polymerisation reactions. However,measurement of the polymer properties during reaction is extremelydifficult. Properties such as the viscosity or melt flow index (MFI) ofthe polymer when melted are very good indicators of the molecularstructure, and hence the physical and chemical properties of thepolymer. However it is necessary either to take samples of polymer fromthe reactor in order to carry out conventional measurements of viscosityand MFI in the laboratory, or in some cases an “on-line” rheometer maybe fitted in the outlet pipe from a reactor.

[0005] Sampling techniques are time-consuming and introduce delays inobtaining the information—therefore this is not an effective way ofcontinuously controlling the reaction since by the time the results areanalysed and appropriate action taken, the reaction conditions will bedifferent and some of the polymer product may already be adverselyaffected.

[0006] On-line rheometers generally work on the principle that a smallamount of molten polymer is syphoned off into a smaller “by-pass” duct,and the rheological properties of the polymer, such as the MFI orviscosity can be measured. The rate of flow of the polymer in theby-pass line at a given pressure (or load) is dependent on the viscosityor MFI of the polymer, at a known shear rate. Hence on-line viscosity orMFI can be measured. Unfortunately, though, this form of measurement istheoretically complicated and involves the use of sophisticated andexpensive equipment for example transducers may be needed to measurepressure and flowmeters and sometimes also gear pumps are required.

[0007] Another approach to this problem is found in GB 2 038 051,published in 1980, which discloses the idea of an “acoustic probe” whichcan be immersed in polymerising mixture in the reactor and used tomonitor the rheological properties of the polymer. The probe wasintended to pick up sound-wave signals from the polymer flowing insidethe reactor, and amongst other things, it was intended to help tomonitor the viscosity of the polymer by correlating viscosity with thelogarithmic decrement of sound-wave oscillations.

[0008] However, in order to pick up useful measurements, the probeneeded to be positioned in a region of polymer flow, such as near to thestirring device in the reactor. This creates practical difficulties inthat the probe is liable to be damaged and is difficult to maintain inposition in the reactor. Any device which has to be immersed in thepolymer melt itself is inherently difficult to operate and is generallybest avoided wherever possible. Furthermore, measurement of polymerproperties in the reactor has problems because the properties in thereactor are not necessarily the same as the properties of the finalpolymer produced.

[0009] In Esbensen et al (1998); “Acoustic chemometrics; from Noise toinformation”, Chemometrics and and intelligent laboratory systems44(1998) 61-76, an acoustic device is described for use with particulatematerials.

[0010] Viewed from one aspect, the invention provides a method ofcharacterising a polymer under test, comprising:

[0011] detecting acoustic emissions from said polymer flowing in aconduit to provide acoustic emission data,

[0012] comparing the acoustic emissions data obtained against acousticemission data from a polymer, or a series of polymers, of knowncharacteristics, and

[0013] thereby characterising the polymer.

[0014] Preferably, said method enables a rheological property of apolymer under test to be determined, by comparing the acoustic emissionsdata against such data from a polymer, or a series of polymers, of knownrheological properties, and thereby determining the rheological propertyof said polymer under test.

[0015] It will be appreciated that in order for the polymer to flow anda meaningful evaluation of its properties to be deduced, it willgenerally be necessary to melt the polymer so that it is no longer insolid form. Hence reference to a “polymer flowing” as used herein shouldbe understood in general as reference to a molten polymer which is ableto flow.

[0016] Thus, the invention is based on the discovery that it is possibleto determine the characteristics, preferably the rheological properties,of a polymer flowing in a conduit, without the need for expensive andcomplex equipment and without the need to immerse a probe or sensor inthe flowing fluid. It has furthermore surprisingly been found that theacoustic emissions from a particular polymer are sufficientlycharacteristic for each different type of polymer to be identified.Also, for any given polymer for which molecular structure may differfrom batch to batch or over time during continuous processing, thisvariation can be monitored. In fact, the composition of the polymer canbe determined from the acoustic emission data of that polymer.

[0017] The composition of the polymer can of course be inferred ordetermined from any values of the rheological properties obtained, e.g.from the viscosity of the polymer, but it will be appreciated thatdirect comparison of emission data alone from polymers of known identitycan also be made. Thus, in order to identify a particular polymeraccording to its composition, a value for the viscosity or otherrheological property of that polymer need not actually be determinedfrom the acoustic emission data in order to identify the polymer.

[0018] Thus viewed from another aspect, the invention provides a methodof identifying a polymer under test, comprising:

[0019] detecting acoustic emissions from said polymer flowing in aconduit,

[0020] comparing the acoustic emissions data obtained against acousticemission data from a polymer of known identity, and

[0021] thereby determining the identity of the polymer under test.

[0022] In this case, the identity of the polymer may be in the form ofan accurate determination of the molecular structure of the polymer, orit may be simply be an indication of the type of polymer being produced(e.g. determining whether it is polyethylene, polypropylene or aparticular co-polymer, or even the particular composition).

[0023] Rheological properties as referred to herein include viscosity(intrinsic, extrinsic, kinematic or dynamic viscosity), shear-strain orshear-stress, melt flow index (MFI) or any other rheological parameterwhich is characteristic of a given polymer. [The term “rheologicalproperty” as used herein however does not include parameters such asflow rate or flow velocity, temperature, pressure, load or pressure dropwhich may or may not be determined incidentally when the method of theinvention is carried out. These and many other properties of a fluidflowing in a conduit are not “rheological properties” within the meaningof the invention since they are not characteristic of any given fluid orpolymer].

[0024] It will be appreciated by those skilled in the art thatrheological properties are generally determined for a given fluid at apre-determined or preferably constant value of the non-rheologicalproperties. Thus for example the viscosity of a fluid may vary withtemperature, flow rate, pressure etc., hence a value of viscosity shouldideally be compared against another at a given temperature and undergiven flow conditions etc. Since it is the molecular weight andmolecular weight distribution (MWD) which is of prime interest incontrolling the properties and hence quality of the polymer product, itis a change in any of these properties which is of interest rather thanmeasurement of an absolute value, in most cases.

[0025] In fact, the viscosity of a polymer also varies with otherrheological properties, e.g. shear stress. If a graph of shear stressagainst viscosity is plotted for a given polymer, the shape of the curveis indicative of the molecular weight distribution of the polymer.However, by comparing the acoustic emission data obtained in accordancewith the invention against emission data from known polymers under thesame flow conditions e.g. at a given temperature, flow rate etc.,complex calculations of the polymer properties can be avoided and theidentity and/or rheological properties of a polymer can be determineddirectly.

[0026] It is therefore preferred that the method of the invention beperformed by detection of acoustic emissions from the polymer at apre-determined flow rate, pre-determined pressure and/or apre-determined temperature. In particular, it is advantageous to controlthe flow rate of the polymer in order that the shear rate of the polymeris known. For example, the flow rate of the polymer may be controlledover a pre-determined range corresponding to a desirable shear raterange for the polymer under test. In this way, it is possible tooptimise the flow rate to provide a shear rate in which the bestpossible distinction in measured characteristics (e.g. viscosity) isobtained for any given polymer. The skilled person will readilyunderstand how to determine the optimal flow rate range by carrying outsimple tests at different measured flow rates. The optimal flow raterange for any given polymer will depend on the characteristic of thepolymer which is to be determined.

[0027] Apparatus to measure the temperature of the polymer in theconduit is well-known in the art and may for example be a thermocoupledevice contained in or placed on the conduit. Alternatively, thetemperature of the conduit in which the polymer flows can be measuredeither at or near to the point at which the acoustic sensor is located,or at another convenient point e.g. at the nozzle outlet of an extruder.All that is required is that the temperature should be pre-determined ata given point which is indicative of (i.e. related to or dependent on)the temperature of the polymer at the point where the acoustic emissionsare being detected.

[0028] In many cases, the temperature at which a polymer melts will besignificantly above ambient temperature. Typically, temperatures of apolymer melt may exceed 100° C. and may be as high as 125 to 250° C. orhigher. The sensor means used to direct the acoustic waves emitted fromthe polymer must therefore in many applications be able to withstandthese high temperature.

[0029] A typical acoustic sensor means for use in accordance with theinvention would be an accelerometer. Accelerometers are known acousticsensor devices and are widely available, for example of the typemanufactured by Brüel and Kjaer in Denmark. Where high temperatures needto be withstood by the sensor means, this should be borne in mind whenselecting a suitable device. Accelerometers for example can bemanufactured to withstand temperatures up to and above 250° C. and thetechnology to do this is well known to manufacturers of accelerometers.

[0030] The conduit in which the polymer flows may take any form.Preferably however the conduit is a pipe e.g. a cylindrical pipe whichmay be made of any suitable material. Steel is typically used in polymerproduction processes but other corrosion-resistant materials may beused. The material of the conduit should however be suitable to allowacoustic waves to be well conducted in order to be detected outside theconduit. Hence acoustically conductive materials, especially metals suchas steel are preferred. The acoustic sensor means must be placed inacoustical contact with the conduit.

[0031] In order to enhance the acoustic emissions from the polymer as itflows, it is preferable to cause a disturbance in the flow of thepolymer in the conduit. For example, the pipe may be modified in someway to alter the flow characteristics, especially to cause a suddenchange in the flow. Thus, a structural detail may be provided in theconduit in order that the conditions of flow change, at or near theportion of the conduit in which acoustic emissions are detected. It hasbeen found that the presence of a constriction in a pipe is particularlysuitable. The diameter of the constriction is not crucial but it must besufficiently small relative to the diameter of the conduit to allow thenecessary degree of turbulence to occur. An orifice plate of the typeroutinely used for flow measurement is an ideal way of providing aconstriction in a pipe. Other forms of structural detail which may beused to create turbulence include, but are not limited to, a bend (e.g.45° or 90°) in the conduit, the presence of a valve or other chokemechanism. A sudden increase in pipe diameter may also be suitable.

[0032] Where the polymer exits the reactor in molten form (e.g. lowdensity polyethylene) the conduit may be an exit pipe directly from thereactor, or it may be a by-pass pipe from one of the main polymerpipelines. Where the polymer is initially in solid form (e.g. granulesor powder) a melting step is needed. The acoustic rheometer inaccordance with the present invention may be used in a similar manner toexisting or known rheometers i.e. it is suitable for use in any form ofconduit and therefore it may simply replace an existing rheometer. Forexample, existing and known rheometers such as online rheometers areoften situated in a by-pass line from an extruder or they may be placedon an extruder directly. For example, the conduit in accordance with theinvention may be associated with a single or plural screw extruder.

[0033] As mentioned above, flow conditions are also preferably kept at apre-determined level in order to allow effective comparison of acousticemission data with data from known polymers. Hence, preferably the flowrate of the polymer in the conduit is measured and/or monitored at ornear the point at which the acoustic sensor is positioned. Flow ratescan conveniently be measured by any method known in the art i.e. by anyflowmeter, but it may in some instances be convenient also to measureflow rates by acoustic means, e.g. by detecting the Doppler shift etc.However, in order to measure the flow rate in accordance with suchapparatus, it will be noted that a sound-wave (ultrasound >25 kHz)source other than the polymer flow itself must be present, as thistechnique depends on detection of ultrasound waves which are reflectedoff the flowing fluid.

[0034] This differs from the detection method of the present inventionwhich relies on passively emitted acoustic waves from the polymeritself. However, there is no reason why any necessary flow ratemeasurements cannot be taken using a separate ultrasound sensor means insender-receiver mode, and utilising this ultrasound sensor means to pickup the reflected ultrasound for flow rate measurement.

[0035] Where the polymer is passing through a pipe or extruder, thepressure in the extruder or pipe is also preferably measured and/ormaintained at a pre-determined level.

[0036] As explained above, the invention relies on the principle thatmovement of the polymer, for example through a constriction in theconduit, causes the polymer-conduit assembly to produce vibrationalacoustic emissions, which can then be detected. One preferred way inwhich the detection takes place is to generate an acoustic spectrumwhich typically may take the form of a graphical representation of theemitted acoustic waves. An example of an acoustic spectrum is shown inFIG. 5. However, in its simplest form, an acoustic spectrum generated inaccordance with the invention could take the form of a plot of amplitudeon the Y axis against frequency on the X axis called a “power spectrum”.

[0037] The acoustic spectrum for any given polymer acts as amultivariant “fingerprint” for that polymer, since it is different fromthe spectra of other polymers (and other fluids generally) flowing atthe same point in the conduit under the same flow conditions. Hence, inaccordance with the invention, a polymer can be identified by comparingits acoustic spectrum against acoustic spectra of known polymers until amatch is found. Where the rheological properties of that polymer arealso known, the rheological properties of the polymer under test canalso be determined from a comparison of the acoustic spectra.

[0038] If the acoustic spectra are recorded e.g. in electronic form orin any other form of searchable database, rapid comparison of data canbe carried out e.g. by computer analysis, and swift matches for theidentity of a polymer and/or the rheological properties of a polymer canbe found. The speed of response which can be achieved using computerprocessing techniques means that data obtained from the detection ofacoustic emissions can be analysed in a database, and values forrheological properties or the identity of a polymer can be determined ina matter of seconds, or even milliseconds. Hence the method of theinvention is particularly advantageous for on-line monitoring ofproperties of polymers and this can be used to facilitate processcontrol.

[0039] Thus in a preferred aspect, the invention provides a method forthe determination or on-line measurement of the rheological propertiesof a polymer, comprising:

[0040] detection of acoustic emissions from said polymer flowing in aconduit, and

[0041] comparison of the acoustic spectrum generated against theacoustic spectra of polymers of known rheological properties, whereby todetermine the rheological properties of the polymer under test.

[0042] The range of acoustic emissions detected may be anywhere in theacoustic frequency range of 0 to about 25 kHz.

[0043] As explained above, the acoustic emissions detected can provide aset of data which can provide a “fingerprint” of the polymer concerned.

[0044] In a simple case, acoustic emission spectra can provide a set ofnumbers which is characteristic of the particular polymer produced. Thisset may be compared with a corresponding set which is known to relate toacceptable products (e.g. from previously produced product). Bydetermining whether the numbers are sufficiently similar (e.g. withinpreviously specified tolerances) it may be determined whether the fluidis itself acceptable. It will be appreciated that these numbers relateindirectly, but unambiguously to molecular weight and molecular weightdistribution, although absolute values need never be found for theseparameters. Nevertheless, it may in practice also be useful to do so.

[0045] The previously acquired sets of acoustic emission data may havebeen obtained by making similar measurements of known polymers havingdesired characteristics. For example, sets of data may be obtained foreach polymer which it is desired to produce corresponding to the idealconditions for producing that polymer.

[0046] Close similarity between the measured data and one of thesepreviously acquired sets of target data may then be used to identify thepolymer concerned and/or to determine whether a desired polymer is beingproduced with the correct characteristics.

[0047] It will be appreciated that this comparison could be performed innumerous ways and in the simplest case useful information could beobtained even from visual comparisons of plots of the various data sets.However, these comparisons are preferably automated. In practice thismeans that the comparisons are carried out by a computer.

[0048] Numerous known computational techniques may be used to performthe analysis, but it is has been found that multivariate calibration isparticularly effective and accurate (see Martens and Naess 1989“Multivariate Calibration” published by John Wiley, Esbensen (1998)).Thus, a latent variable corresponding to an optimal linear combinationof the measured frequency data may be introduced. The data are thenredefined in relation to this latent variable.

[0049] In a particularly preferred form of the invention, PrincipalComponent Analysis (PCA) of the acoustic emissions data is used forclassification of new samples in relation to old samples of knownproperties. The raw data may be subjected to preprocessing such as e.g.transformation, centering, smoothing or scaling. Subsequently, from aset of samples (“calibration set”) of known properties a data subspaceis empirically identified into which the test sample data points may beprojected. This subspace is described by a set of “latent variables”,spanning individual axes in the subspace and is denoted the “model” ofthe given class of samples. The number of latent variables are thenempirically found as those needed to give representative informationrelated to flow properties of the fluid in question based on casualknowledge by the operator. It will be noted that it is not necessary torun any transformation to align with rheological parameters.

[0050] If a visual evaluation is desired, a plot of the data may beproduced where the axes are given by the latent variables, and where newsamples are compared to the set of known samples, and to limiting valuesbased on the same samples. For a mathematical evaluation(classification) upper and lower limiting values may be defined for thevalue of the latent variables, and for residuals of the raw data afterprojecting into the subspace an upper limiting value is defined. Thennew samples may then be classified according to these limiting values.This approach has been termed the SIMCA approach, as referred to inEsbensen 1998 and numerous other references herein.

[0051] Typically, when using PCA, the latent variables are defined bythe eigenvectors of the (n×k) matrix e.g. where n is the number ofsamples in the calibration set and k is the number of values measuredfor a given variable. Each sample in the calibration set, and futuretest process samples, may then be described by their score values alongthe individual latent variables thus defined.

[0052] By calculating the correlation of the latent variable withpolymer property parameters like MWD, MFR (melt flow rate), etc. onewill obtain knowledge of along which direction these parameters havetheir largest variability in the latent variable data space. Thisinformation may be compared to the position of the individual samples inthe same data space, to evaluate their score in relation to thedifferent parameters.

[0053] By calculating the correlation of the latent variable withprocessing parameters like reactor temperature, reactor feedcompositions etc., one will obtain knowledge of along which directionthese parameters have their largest variability in the latent variabledata space. This information may be compared to the position of theindividual samples in the same data space, to evaluate their score inrelation to the different parameters, and it may be used to estimate howprocess parameters should be changed to change the positioning of theproduct in the latent variable space to have the selected flowproperties represented by the acoustic emission data values.

[0054] It is particularly preferred for the method to be implementedusing a computer arranged to display a score plot representing the dataat least substantially in real time. In this way, as new data isacquired and new plots are added to the score plot, changes in the fluid(polymer) characteristics may be followed. It is helpful for anindication to be provided on the display of where the boundaries betweenacceptable and unacceptable points lie, for example based on statisticalquantities. The indications may be a boundary line in the form of anellipse. Points falling outside the boundary correspond to unacceptableproduct.

[0055] As discussed above, the score may be evaluated in relation todifferent parameters and so it is possible to correlate the position ofa point outside the boundary with the corresponding deficiency in itsproperties. This information may then be used to enable appropriatecorrective action to be taken by a plant technician. For example, thepreviously acquired data sets could include data corresponding to knownincorrect settings for the desired product from which previouslydetermined corrective action may be taken. Such previous data sets couldhave been deliberately produced or they could be learned automaticallyfrom analysis of previous operations of the plant. Alternatively theplant may be adjusted in an iterative manner based upon the nature ofthe deviation of the measured data sets from the desired data set.

[0056] In particularly preferred forms of the invention, means isprovided to automatically adjust the operating conditions of the plantin order to ameliorate the deficiency. Of course, there need not be adisplay for this to be effective—the “ellipse” may simply be a definedvolume of data space.

[0057] Another advantage of this form of the invention is that even if aproduct is determined to be acceptable, it is possible to monitorvariations in where points are plotted (or located in data space) inorder to determine trends which may be used to anticipate futuredeficiencies and to take corrective action before they occur. Preferablythis is also implemented automatically.

[0058] In this context PCA represents one way of identifying the latentvariables. However, it will be appreciated that any other mathematicalmethod involving linear or non-linear transformation of the relevantprocess data into a set of latent variables may be used. Examples ofother methods are Partial Least Squares Regression (PLSR), NeuralNetworks (NN) and curve fitting of the pressure data or preprocessedpressure data to a curve of selected exponential degree.

[0059] A particularly preferred aspect of the invention is to use theacoustic emission data for quantification of selected polymerproperties, e.g. MFR or MWD. Again the raw data may be subject topreprocessing such as e.g. transformation, centering, smoothing orscaling. From a set of samples (“calibration set”) of known propertiesit is then possible empirically to identify a mathematical relation (the“model”) to quantify the selected properties based on the preprocessedpressure. This model may be any linear or non-linear relation defined bymethods like Principal Component Regression (PCR), Partial Least SquaresRegression (PLSR), Neural Networks (NN), etc.

[0060] When using PCR and PLS, latent variables may be identified in amodified form closely related with PCA (above), and then a linearregression model is developed between the polymer property and this typeof latent variable. In the same way as when doing classification above,the score values in the latent variable space may then be used forvisual and mathematical evaluation. Correlation between the latentvariables and process parameters may be used to identify how the processparameters should be changed to adjust the selected property of thepolymer being produced.

[0061] It will be appreciated from the foregoing that the presentinvention is useful in the field of polymer production and so acousticemission detection means is preferably situated on-line and may beassociated with an extruder used in such a context. Polymer may be fedfrom the extruder directly into a suitably modified conduit for acousticemissions to be detected, e.g. by means of a bypass. Because of thespeed of operation and the improved accuracy of the method of theinvention, if the properties of the polymer are as desired, this will beknown much more speedily than in the prior art system. Furthermore, itis also possible to determine more quickly if the measuredcharacteristics are not as required and then to adjust the operatingconditions of the reactor accordingly in order to obtain the desiredcharacteristics. Consequently, wasted production may be greatly reduced.

[0062] It is possible to apply the method of the present inventioneither only when the reactor is first set up for a given production run,or at occasional intervals as required by quality control. However,since the method may operate automatically it is particularly preferredthat regular and comparatively frequent measurements be made, say aroundevery 10 minutes.

[0063] Polymer producing plants are normally operated continuously andif it is desired to change from production of one polymer to anotherthis is done without closing down the plant. Instead, the reactoroperating conditions are adjusted in order to change the polymer therebyproduced and fed to the extruder. Thus, preferably the method of theinvention is used to obtain data which is used to monitor the transitionbetween products. Since in the preferred forms of the invention the dataacquisition and subsequent comparison steps are carried out by computer,this may be done rapidly. Consequently, the transition may be effectedmore smoothly and quickly than in the prior art and moreover theoperator can determine more quickly when the desired product starts tobe produced. It will be appreciated that this significantly reduces theamount of wastage associated with operation of the reactor therefore asignificant advantage in terms of saving time and materials and therebycosts.

[0064] The invention also provides an apparatus, also referred to hereinas an acoustic rheometer, for carrying out the method of the invention,and the use of the acoustic rheometer to control a polymerisationreaction. Thus viewed from a further aspect the invention provides anapparatus for characterising a polymer, comprising:

[0065] a) an acoustic sensor capable of detecting acoustic emissionsfrom the polymer and thereby generating a signal;

[0066] b) means for comparing the signal against acoustic emissions datafrom polymers of known characteristics. This data may for example bestored in a computer memory either provided within the apparatus orremotely.

[0067] The invention also provides the use of an acoustic rheometercomprising

[0068] a) an acoustic sensor capable of detecting acoustic emissionsfrom a polymer;

[0069] for controlling a polymerisation reaction producing said polymer.Preferably, in this aspect, the acoustic rheometer further comprisesmeans for comparing the signal against acoustic emissions data frompolymers of known characteristics, as defined above.

[0070] Preferably, the apparatus is adapted for determining therheological properties of a polymer, comprising:

[0071] a) an acoustic sensor capable of detecting acoustic emissionsfrom the polymer and thereby generating a signal;

[0072] b) means for comparing the signal against acoustic emissions datafrom polymers of known rheological properties whereby to determine avalue for the desired rheological property of the polymer under test.

[0073] The apparatus may further comprise means for identifying thepolymer. Preferably, the apparatus comprises an acoustic sensor which iscapable of detecting vibrational acoustic emissions in the interval 0-25kHz.

[0074] The acoustic sensor may be as described above. The means forcomparing the signal (referred to hereinafter as “comparison means b)”)may if necessary or desired comprise means for amplifying or processingthe signal from the acoustic sensor. For example the comparison means b)may be a computer which in turn may be connected e.g. to a signalamplifier or preprocessor. The computer will preferably be loaded withsuitable software. Conveniently, the comparison means b) may be providedby a package such as the Multi-Channel Spectrum Analyser available fromApplied Chemometrics Research Group (ACRG), Tel-Tek, Porsgrunn, Norway.

[0075] The apparatus of the invention is set up such that the acousticsensor means is positioned in acoustic contact with (preferablytouching) the conduit through which a polymer can flow. The conduit ispreferably a straight segment of a pipe and preferably this has astructural detail as hereinbefore described. The acoustic sensor meansis therefore positioned whereby to detect acoustic emissions from theflowing polymer as it passes through the structural detail in the pipe.

[0076] It has been found in particular that the acoustic sensor meanscan be placed in a variety of positions in relation to the conduit inorder to successfully determined rheological properties of a polymer.For example, it could be placed before or after the structural detaile.g. within about 5-20 cm or 5-10 cm of the structural detail (relativeto the direction of flow) but preferably it should be placed before thestructural detail. Alternatively it could be positioned at the positionof the structural detail itself, which is particularly preferred.

[0077] The invention also extends to a polymer production plantincorporating the method or apparatus of the invention as set forthabove and also to polymer products thereby produced.

[0078] Certain embodiments of the invention will now be described, byway of example only and with reference to the accompanying drawings inwhich:

[0079]FIG. 1 is an acoustic rheometer according to the invention;

[0080]FIG. 2 is a schematic flow diagram showing the data path foranalysis of the acoustic emissions data;

[0081]FIG. 3 is a diagram of one possible configuration of an acousticrheometer according to the invention;

[0082]FIG. 4 is another diagram showing a different possibleconfiguration of an acoustic rheometer according to the invention;

[0083]FIG. 5 shows an acoustic spectrum in the frequency range 0-25 kHz,for each of the four different polymers as described in Example 1.

[0084]FIG. 6 shows a score plot acoustic spectrum for PCA (principalcomponent analysis) of these four polymers. The percentage score ofComponent 2 (22.3%) is plotted against the percentage score of Component1 (35.7%).

[0085]FIG. 7 shows the PLS model (partial least squares), with theviscosity of the modelled values plotted against the measured viscosityfor each of the polymers.

[0086]FIG. 8 shows the variation in viscosity of the four differentpolymers as measured by a rheometric dynamic analyser in a frequencysweep mode (190° C. melt temperature). On the Y axis the crossplotviscosity is given at 300 rad/sec and on the X axis it is given at 0.05rad/sec.

[0087]FIG. 9 shows a score plot (t1t2) of five replicates of each of thepolymers designated A, B, C and D.

[0088]FIG. 1 shows an ultrasound rheometer for operation in accordancewith the invention. Typically, the polymer melt leaving thepolymerisation reactor will be processed through an apparatus 1 whichconsists of a conduit 2 with a constriction 3 allowing the polymer topass through. The acoustic sensor means 4 e.g. an accelerometer may beplaced in any one or more of positions A (before the constriction), B(at the constriction) or C (after the constriction). The accelerometer 4detects acoustic emissions from the polymer flowing through theapparatus 1 and generates an acoustic spectrum which is characteristicof the polymer. The signal is amplified by an amplifier/preprocessor 5and data analysis is carried out by a computer 6 or other suitablemeans. Data analysis can for example be carried out by mulitvariateanalysis techniques such as principle component analysis (PCA) orpartial least squares (PLS). Information on the viscosity, molecularstructure, MFI and other polymer properties can then be calculated bycomparison with information from known polymers.

[0089]FIG. 2 is a schematic flow diagram showing the data path from theacoustic emissions (vibrations) generated by the polymer and how thatdata is analysed numerically. Box 7 represents polymer flow through theconstriction in the conduit, from which the acoustic signal is detectedby the sensor accelerometer 8. Box 9 represents signal processing byadaptation of the signal through a lowpass filter and analog-digitalconversion to allow analysis of the signal. Multivariate analysis of thesignal data is then carried out, as represented in box 10.

[0090]FIG. 3 shows one possible configuration for the acoustic rheometermounted on a by-pass from an extruder. The extruder barrel 11 is shownwith the by-pass line 12 leading from it and round the by-pass “loop”back into the extruder 11. Polymer is pumped round the by-pass pipe 12by means of a gear pump 13, piston, or any other suitable device forgenerating flow, and through a constriction 14 in the by-pass pipe. Theacoustic sensor means 15 is placed outside the pipe in acousticalcontact therewith, and leads 16 transmit the signal to an amplifier 17and then to a personal computer 18 which is capable of analysing thedata by means of multivariate analysis (MVA).

[0091]FIG. 4 shows another possible configuration for an acousticrheometer. The polymer process flow from the polymerisation reactor isin powder form and is transported through pipe 19 from the reactor. Aportion of the polymer is drawn off from the main flow pipe at asampling point/system 20 and passed through a single screw extruder 21or any other suitable device where it is heated and melted to allow itto flow. An acoustic sensor means 22 is placed in acoustical contactwith the single screw extruder pipe at a point before where the polymerflows through a constriction 23. The signal detected by the acousticsensor means 22 is transmitted to a data analysis unit such as acomputer (not shown).

EXAMPLE 1 Comparison of 4 Different HDPE Resins Using Acoustic Rheometer

[0092] The aim with this study was to compare the acoustic spectrumrecorded as described in the patent with viscosity data obtained using aconventional rheometer (plate—plate dynamic rheometer; Rheometricsdynamic spectrometer, RDA-II)

[0093] For this purpose 4 commercial HDPE (high density polyethylene)polymers manufactured by Borealis were chosen:

[0094] HE8168, HE8343, LE7520, LE0400

[0095] Viscosity data obtained by the dynamic rheometer (at 190° C.) isshown in table 1 below: TABLE 1 Viscosity vs. shear rate at 190° C.viscosity (Pa.s) at different shear rates polymer shear rate LE7520LE400 HE8343 HE8168 25 511 1986 3793 860 38 419 1516 2973 857 50 3681269 2534 855 74 306  985 2017 840

[0096] The experimental setup for the rheometer is shown in FIGS. 1 and2;

[0097] The rheometer is basically a heated pipe in which a die isinserted in order to create a constriction in the pipe. At the flowinlet of the die is placed an accelerometer in order to record soundgenerated by the flowing polymer. Polymer is being fed by a 30 mmextruder (manufactured by company Collin GmbH)

[0098] The procedure of collecting and numerically treating the data isshown schematically in FIG. 2. (for further explanation refer toEsbensen et al (1998); “Acoustic chemometrics; from Noise toinformation”, Chemometrics and and intelligent laboratory systems44(1998) 61-76.

[0099] Each polymer was extruded at 4 rpm's (30,45,60, 90 ). With thedie chosen for the experiments (7 mm diameter) these rates equal shearrates as shown in table 1. In table 1 (above) viscosities for the 4polymers at the given shear rates are given based on laboratorymeasurements.

[0100] During the experiment with the acoustic rheometer, the followingdata were recorded:

[0101] polymer temperature (end of extruder)

[0102] polymer pressure

[0103] Polymer temperature at the measurement point

[0104] acoustic spectrum (FIG. 5)

[0105] MVA Analysis

[0106] A plot of the different acoustic spectra is shown in FIG. 5

[0107] A PCA (principal component analysis) analysis is shown in FIG. 6:the scores of the first two latent variables show that the spectra areable to distinguish between the different polymers in a systematicmanner.

[0108] By combining table 1 and the recorded spectra we can use PLSregression technique to study how the acoustic spectra explain thevariation on viscosities at the actual shear rates.

[0109] The PLS model (FIG. 7) explains the measured viscosities by 94%in 2 comp\ 96% in 3 com. .Cross validation reduces the explainedvariance to around 60%.

[0110] From this it can be concluded that the recorded spectra at agiven flowrate relate to one point on the dynamic spectra curve. As donein this experiment, running at four different flow rates (or using 4different dies) one can put the spectra together to characterize theflowcurve of the polymer.

EXAMPLE 2 Study Lot Variation Within a Single Product by Use of AcousticRheometer

[0111] It is well known that any commercial polymerisation process willbe subject to certain variations in the molecular structure of thepolymer produced. The amount of variation is usually low and in somecases difficult to quantify. Online methods are used to measure thisvariation in properties. The accuracy of the online rheometer willdetermine how well small variations can be detected and thus in the longrun avoided. To test the acoustic rheometer of the invention, 4different lots of a polymer grades with known difference in molecularstructure were tested using the same setup as in example 1.

[0112]FIG. 8 shows the variation in viscosity of the 4 lots as measuredby means of a rheometrics dynamic analyser in a frequency sweep mode(190° C. melt temperature).

[0113] Each sample was extruded at 4 rates (30,50,70.100 rpm on the 30mm extruder). Spectra in the range 0-25 KHz were recorded on a Bruel &Kjaer acclerometer (nr 4384) TABLE 2 Part-list (high temperatureequipment) 1 Accelerometer 250° C. Brüel & Kjaer, Number: 4384 Denmark 2Coaxcables, 2 mm, Brüel & Kjaer, Number: AO 0038 250° C. Denmark 1Charge/Deltatron Brüel & Kjaer, Number: 2646 ampl. Denmark 1 UNF to BNCadapter Brüel & Kjaer, Number: JP 0145 Denmark 25 Cement studs Brüel &Kjaer, Number: UA 0866 Denmark 25 Extension Brüel & Kjaer, Number: UA0186 connectors Denmark

[0114] TABLE 3 Recording Parameters: Sampling frequency: 50 kHzFrequency range: 0-25 kHz Number of variables: 512 fewq. + 1 Temp. = 513variables window size: 1024 data-points Transformation window BlackmanHarris type: Number of replicates: 5 Recording length each 0.02 sec.replicate: Averages each replicate 100 spectrum unit: dBV rms

[0115] Data were pretreated as shown in FIG. 2 example 1. FIG. 9 shows ascore plot of results: for each resin five replicates were run.

[0116] The data show the method to be able to separate betweenindividual lots from a commercial polymerisation.

1. A method of characterising a polymer under test, comprising:detecting acoustic emissions from said polymer flowing in a conduit toprovide acoustic emission data, comparing the acoustic emissions dataobtained against acoustic emission data from a polymer, or a series ofpolymers, of known characteristics, and thereby characterising thepolymer.
 2. A method as claimed in claim 1 wherein a rheologicalproperty of the polymer under test is thereby determined.
 3. A method asclimed in claim 1 or claim 2 wherein the identity of the polymer undertest is thereby determined.
 4. A method as claimed in claim 2 whereinthe rheological property under test is the viscosity of the polymer. 5.A method as claimed in any preceding claim wherein the acousticemissions are detected by means of an accelerometer.
 6. A method asclaimed in any preceding claim wherein said conduit is associated with apipe leading directly from a polymerisation reactor.
 7. A method asclaimed in any preceding claim wherein said conduit is associated withan extruder.
 8. A method as claimed in any preceding claim wherein saidconduit comprises a structural detail.
 9. A method as claimed in anypreceding claim wherein the acoustic emissions data is analysed usingPrincipal Component Analysis (PCA) techniques.
 10. An apparatus forcharacterizing a molten polymer, comprising: a) an acoustic sensorcapable of detecting acoustic emissions from the polymer and therebygenerating a signal; and b) means for comparing the signal againstacoustic emissions data from molten polymers of known characteristics,said apparatus being arranged to thereby characterise said polymer. 11.An apparatus as claimed in claim 10, wherein said apparatus is adaptedwhereby to determine a value for the desired rheological property of thepolymer under test.
 12. An apparatus as claimed in claim 10, whereinsaid apparatus further comprises means for identifying the polymer. 13.Use of an acoustic rheometer comprising a) an acoustic Bensor capable ofdetecting acoustic emissions from a polymer for controlling apolymerisation reaction producing said polymer.